The present invention relates generally to two piece fasteners and more particularly to improved flush head shear type two piece fasteners.
Two piece flush head shear type fasteners typically include a pin having a frusto conically shaped head at one end of an elongated shank which is designed to fit within a suitably prepared countersunk opening in workpieces to be joined. The shank portion of such fasteners generally includes a plurality of lock grooves projecting outwardly beyond the workpieces, a separable pintail portion on the outer end thereof normally having a plurality of pull grooves provided thereon and a collar adapted to be swaged into engagement with the lock grooves.
Such fasteners are well known in the art and are designed to be installed or set by an installation tool which includes a jaw assembly engageable with the pull grooves of the pintail portion and a swaging anvil engageable with the collar. As the tool is actuated the jaws exert a pulling action on the pin and the swaging anvil applies the reaction force to the collar. During the initial or primary clinching operation, the workpieces are first drawn together and the pin drawn into and fully seated within the prepared opening. As the relative free movement between the pin and collar is taken up continued pulling action of the tool will increase the tensile loading on the pin and thereby produce an increasing reactionary force on the collar. As these forces increase, the swaging anvil will operate to swage the collar into the outer lock grooves on the pin thereby providing a primary clinch or lock therebetween and preventing any further relative free movement between the pin and collar. Thereafter, the continued swaging of the collar causes an axial elongation thereof forcing the other end of the collar to exert a clamping force on the workpices and resulting in increased tensile loading on the pin. This increased pin loading due to collar elongation is commonly referred to as secondary clinch loading. Thus, during installation or setting of the fastener, the preformed head provided on the pin is subjected to the combined loading from the action of the installation tool and the secondary clinch loading.
A number of factors influence the actual magnitude of these combined loading forces several of which also directly affect the strength of the installed fastener such as for example collar hardness. In applications requiring relatively high strength it is desirable to use as hard a collar as possible in order to increase the clamping force on the workpieces as well as to increase the fatigue life and improve tensile strength characteristics. However, as collar hardness increases, the required installation loads also increase thus resulting in increased loading on the fastener and the preformed head. Thus, as collar hardness increases these combined forces may exceed the rated head strength of the pin resulting in a partial yielding of the head and degradation of the resulting joint strength. Typically, such degradation is physically evidenced by a dishing or dimpling of the otherwise relatively flat surface of the pin head. Such occurrences are most commonly experienced in conjunction with setting of the shear type fasteners which have a substantially smaller head relative to pin diameter. Therefore, it appears that the head size presents a limiting factor as to the maximum collar hardness which may be used in conjunction therewith and therefore limits the tensile strength and clamping forces which may be obtained. It is not practical nor commercially desirable to design different size preformed heads for fasteners of the same nominal shank diameter for varying collar hardness as each head size would then require separate hole preparation tools to be used in order to assure proper sizing of the countersunk opening.
The present invention, however, provides an improved fastener having a raised head portion of a dimension proportioned to the anticipated peak installation load resulting from both installation tool loading and secondary clinch loading and proportioned to the relative shear strength and diameter of the pin so as to substantially reduce or eliminate the dishing of the fastener head during setting thereof and provide a fastener having a substantially flat flush head when set. Further, the raised head portion results in a pin having a head of increased shear strength thus enabling use of collars of increased hardness resulting in a higher tensile strength fastening system while allowing existing standard hole preparation tools to be utilized. Also, because the frequency of rejected head installations is substantially reduced, cost savings will be realized from the reduction in expenditure of materials and labor attendent with the removal and reinstallation of rejected fasteners.
Further, the head design of the present invention may provide a positive indication or properly installed fasteners. More specifically, because the dimensions of the raised surface portion of the head may be controlled so as to require a predetermined loading in order to flatten out this raised surface portion, the degree of flatness will be proportionate to this loading and also to the degree of clamping force the workpieces are subjected to due to secondary clinch loading. Thus, insufficient "flatness" may provide an indication of inadequate clamping or preloading of the workpieces which may result in premature fatigue failure whereas excessive "flatness" or even dishing may indicate excessive loading and possible damage to the workpieces within the area surrounding the joint as well as reduced tensile strength due to head degradation.
Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings.